1. Field
This patent specification relates to analyzing subsurface electromagnetic survey data. More particularly, this patent specification relates to methods and systems for analyzing electromagnetic survey data obtained using at least one borehole-deployed receiver to generate an image indicating spatial locations of one or more features in a subterranean formation.
2. Background
Cross borehole (CB), surface-to-borehole (STB), and borehole-to-surface (BTS) electromagnetic (EM) imaging surveys (considered hereinafter as controlled-source electromagnetic (CSEM) data) include a series of electric or magnetic dipole sources that are energized in a borehole, on the surface of the earth, or near the seafloor, and measurements of magnetic and/or electric fields are made in a different borehole, or on the earth's surface or seafloor.
As CSEM surveys increase in demand and size and designs become more complex, there is a need to process and interpret recorded data more quickly and efficiently, perhaps even in real-time. The most common approach to reconstructing subsurface conductivity distributions is CSEM inversion, which results in accurate subsurface models but has timing, stability, and computation limitations.
In the past thirty years, downward continuation imaging methods have been used predominantly for seismic reflection problems. See e.g., Biondi, B, 2003, Equivalence of source-receiver migration and shot-profile migration, Geophysics, 68, 1340-1347 (hereinafter “Biondi 2003”), Claerbout, J. F., 1970, Course grid calculations of wave in inhomogeneous media with application to delineation of complicated seismic structure, Geophysics, 35, 407-418 (hereinafter “Claerbout 1970”), Claerbout, J. F., 1976, Fundamentals of Geophysical Data Processing, McGraw-Hill, New York, Chpts. 10-11, Ferguson, R. J., & Margrave, G. F., 2003, Prestack depth migration by symmetric non-stationary phase shift, Geophysics, 67, 594-603, Gazdag, J., 1978, Wave equation migration with the phase shift method, Geophysics, 43, 1342-1351, Loewenthal D., & Mufti, I. R., 1983, Reversed time migration in spatial frequency domain, Geophysics, 48, 627-635, and Stolt, R. H., 1978, Migration by fourier transform, Geophysics, 43, 23-48.
However, there are several early studies that formulate this imaging problem for broadband passive-source EM data. See, Lee, S., McMechan, G. A., & Aiken, C. L., 1987, Phase-field imaging: The electromagnetic equivalent of seismic migration, Geophysics, 52, 678-693 (hereinafter “Lee 1987”), Levy, S., Oldenburg, D., & Wang, J., 1988, Subsurface imaging using magnetotelluric data. Geophysics, 53, 104-117 (hereinafter “Levy 1988”), Madden, T., 1988, Inversion of low-frequency electromagnetic data, Course Notes, 9, Dept. of Earth, Atmospheric, and Planetary Sciences, MIT, 379-408, Velikhov, Y. P., Zhdanov, M. S., & Frenkel, M. A., 1987, Interpretation of MHD-sounding data from the Kola Peninsula by the electromagnetic migration method, Physics of the Earth and Planetary Interiors, 45, 149-160, and Zhdanov, M. S., Traynin, P., & Booker, J. R., 1996, Underground imaging by frequency-domain electromagnetic migration, Geophysics, 61, 666-682 (hereinafter “Zhadanov 1996”). All these papers are applied to the magnetotelluric (MT) problem, while additional works describe the use of EM imaging for broadband time-domain electromagnetic soundings. See, Zhdanov, M. S. & Frenkel, M. A., 1983, The solution of the inverse problems on the basis of the analytical continuation of the transient electromagnetic field in reverse time, Journal of Geomagnetism and Geoelectricity, 35, 747-765, Zhdanov, M. S., Traynin, P., & Portniaguine, O., 1995, Resistivity imaging by time domain electromagnetic migration (TDEMM). Exploration Geophysics 26, 186-194, and Zhdanov, M. S., & Portniaguine, O., 1997, Time-domain electromagnetic migration in the solution of inverse problems. Geophysical Journal International, 131, 293-309. For a discussion of the application of narrow-band downward continuation imaging to surface CSEM data interpretation for land surveys, see, Velikhov, Y. P., Zhdanov, M. S., & Frenkel, M. A., 1987, Interpretation of MHD-sounding data from the Kola Peninsula by the electromagnetic migration method, Physics of the Earth and Planetary Interiors, 45, 149-160. Also see, Tompkins, M. J., 2004, Marine controlled-source electromagnetic imaging for hydrocarbon exploration: Interpreting subsurface electrical properties, First Break, 22, 45-51 (hereinafter “Tompkins 2004”) for a discussion to marine surveys. In the past several years, there have been additional studies on the use of downward continuation imaging to the marine CSEM problem (Rosten, T., Hogstad, K., & Arntsen, B., 2006, 3D depth migration operators for marine controlled-source electromagnetic data, SEG Abstracts, 25, 770-774, and Tompkins, M. J., 2005, The role of vertical anisotropy in interpreting marine controlled-source electromagnetic data, SEG Expanded Abstracts, 24, 514-517 (hereinafter “Tompkins 2005”), and Zhdanov, M. S., & Wan, L., 2005, Rapid seabed imaging by frequency domain electromagnetic migration, SEG Expanded Abstracts, 518-521 (hereinafter “Zhdanov 2005”)). In contrast to analytic continuation methods that use plane wave extrapolation to propagate the wave fields into the earth, fullwave imaging methods have also been applied to land (Zhdanov, M. S., Traynin, P., & Portniaguine, O., 1995, Resistivity imaging by time domain electromagnetic migration (TDEMM). Exploration Geophysics 26, 186-194, and Zhdanov, M. S., & Portniaguine, O., 1997, Time-domain electromagnetic migration in the solution of inverse problems. Geophysical Journal International, 131, 293-309) and marine (see, Mittet, R., Maao, F., Aakervik, O. M., & Ellingsrud, S., 2005, A two-step approach to depth migration of low-frequency electromagnetic data, 24, 522-525 (hereinafter “Mittet 2005”)) CSEM data).
Two patents teach methods that relate to CSEM imaging (i.e., migration, holography). They are U.S. Pat. No. 7,191,063 (“the '063 patent”) and U.S. Pat. No. 6,253,100 (“the '100 patent”). The '100 patent discusses steps in a method of surface CSEM imaging for broadband signals including time-domain and frequency-domain transmissions. Specifically, the '100 patent discusses the steps of incident and scattered field separation, signal propagation in a medium, and application of an imaging condition (cross-correlation). The '100 patent further teaches CSEM imaging using fullwave formulations of both electric and magnetic fields. The '063 patent discusses downward continuation imaging for narrow-band marine CSEM data using between 3 and 15 frequencies. Both the '100 patent and the '063 patent discuss iterative imaging methods. Although not disclosed in a patent, Mittet 2005, discusses an application of fullwave imaging to narrow-band surface CSEM data using a modified imaging condition. However, none of these patents or publications discuss CSEM imaging methods applied to cross-well (xwell), borehole-to-surface (BTS), or surface-to-borehole (STB) configurations.